Bio-Control of Salvinia

Salvinia (Salvinia molesta), an attractive floating water fern native to southeast Brazil, has become a weed in many parts of Asia. It spreads rapidly across fresh water surfaces, hindering travel on rivers, invading rice paddies and upsetting the underwater habitats beneath it. With the water hyacinth, salvinia is one of the world’s two worst water weed problems.

Since it was introduced to the Old World around 1940, salvinia has infested fresh water bodies in Australia, Papua New Guinea, India, Sri Lanka, Malaysia, Singapore, Indonesia, the Philippines and parts of Africa. Able to double in size in a little over two days, it forms thick mats that block movement of small boats, prevent access to drinking water, clog irrigation ditches and cut off light to underwater organisms, killing fish and the organisms on which they feed. Its widespread range and fast growth make herbicide and physical control measures impossible and, since the plant is 96% water by weight, it cannot be cheaply used as a processed animal feed.

With no natural enemies, the floating fern proliferated until Australian scientists identified a Brazilian beetle (Cyrtobagous salviniae) whose adults feed on growing tips of the plant and whose larvae invade its rhizomes. In 1980, Dr. Peter Roon and colleagues from the CSIRO Department of Entomology released C. salviniae beetles in Australian waterways, commencing a biocontrol program that reduced salvinia populations to a few scattered individuals within 24 months.

Shortly thereafter, Dr. Roon's team focused its biocontrol efforts on Southeast Asia. The researchers introduced C. salviniae to the Sepic River system in Papua New Guinea, achieving the same dramatic results as in Australia. Still later, in 1986, they responded to Sri Lankan requests for assistance. After a quarantine period to determine if the beetle would attack native plants, adult beetles were released in salvinia-choked Sri Lankan waterways. In early 1987 the beetles had established colonies, and by September salvinia populations were eradicated in the Battulu Oya region.

Extensive tests have demonstrated that the C. salviniae beetle will not significantly damage crops or other Asian plants in its habitat. In countries using azolla as a manure for rice, there was concern that the beetle would destroy this, too. In the absence of salvinia it will ‘nibble’ on azolla, but not on the growing tips. Experiments demonstrate that it only breeds on salvinia as the rhizomes of the azolla are too small for the larvae. One word of caution, eradication of salvinia may bring about infestations of water hyacinth (Eichhornia crassipes), which occupies similar ecological niches.